1. Field of the Invention
The present invention relates to an inorganic scintillator and to a process for its fabrication.
2. Related Background of the Invention
In an apparatus used for Positron Emission (computed) Tomography (hereinafter, “PET”), a massive scintillator is mounted for highly efficient detection of 511 keV γ rays emitted from a specimen injected with a radioactive agent. The optical characteristics (wavelength conversion characteristics, etc.) of the scintillator mounted in the PET apparatus have a major effect on the imaging performance of the apparatus, and therefore improvement in the optical characteristics of the scintillator is the most important means of enhancing the imaging performance of such apparatuses. Researchers are therefore actively exploring scintillator materials which can be used to construct scintillators with excellent optical characteristics, and are developing manufacturing techniques such as crystal growth techniques for realizing such scintillators.
In the field of high-energy physics as well, experiments for detection and analysis of high-energy microparticles arriving to earth from outer space require implementation of scintillators which allow efficient detection of high-energy microparticles.
Scintillators mounted in PET apparatuses must exhibit various aspects of performance including a large time-integrated value for the outputted scintillation pulse intensity (hereinafter, “scintillation output”), short scintillation rise times and scintillation decay times, high energy resolution and high density. Particularly for higher efficiency detection of γ rays emitted from subjects being examined by PET, it is preferred for scintillators to produce a greater scintillation output. Also from the standpoint of alleviating the burden on subjects, the examination time per subject must be shortened and therefore scintillators with short scintillation decay times are desired.
The time-dependent change in the intensity of a scintillation pulse outputted upon incidence of a radiation into a scintillator will now be explained. FIG. 1 is a graph which schematically illustrates typical time-dependent change in the intensity of a scintillation pulse. The scintillation pulse rises relatively precipitously to the maximum intensity value Imax and gradually decays thereafter. Throughout the present specification, “scintillation output” refers to the time-integrated value of the scintillation pulse intensity from the time at which the scintillation pulse intensity (scintillation intensity) is at Imax (tmax) to the time at which scintillation is no longer observed (the shaded section in FIG. 1). Also, “scintillation decay time” refers to the time required from the maximum value of the scintillation pulse intensity (Imax) during the rising stage of the outputted scintillation pulse to Imax/e (where e represents the base of the natural logarithm).
Inorganic scintillators made of inorganic materials are currently employed as scintillators for PET, and as examples there may be mentioned those employing Bi4Ge3O12 (hereinafter “BGO”), Gd2SiO5 (hereinafter, “GSO”) and Lu2SiO5 (hereinafter, “LSO”) as matrix materials. BGO is used in a PET apparatus (trade name: “Discovery”) by GE Healthcare Corp., GSO is used in a PET apparatus (trade name “ALLEGRO”) by Philips Medical Systems, and LSO is used in a PET apparatus (trade name: “ECAT ACCEL”) by Siemens Corp.
The scintillation output for scintillation produced upon incidence of a radiation to the scintillator, as one of the luminescent properties of these inorganic scintillators, is a relative value of 2.0 for GSO and 4.0 for LSO with respect to 1.0 for BGO. The scintillation decay time is approximately 300 ns for BGO, approximately 60 ns for GSO and approximately 40 ns for LSO. GSO and LSO which exhibit superior optical characteristics among these inorganic scintillators have structures wherein Ce has been added as a luminescent center in a matrix material composed of the aforementioned rare earth element-containing metal oxides
It is known that among inorganic scintillators having structures wherein Ce has been added as a luminescent center in a matrix material composed of a rare earth element-containing metal oxide, inorganic scintillators which employ Gd-containing metal oxides tend to have two or more scintillation components exhibiting different scintillation decay times (see, for example, IEEE Transactions Nuclear Science, Vol. 37, No. 2 (1990) 161 and Journal of Physics: Condensed Matter 7(1995) 3063). In addition, it has been demonstrated that the scintillation decay times of the scintillation components tend to be shorter when the value of the total number of Ce atoms in the scintillator divided by the total number of rare earth element atoms (hereinafter, “Ce content ratio”) is high (see, for example, Japanese Patent Application Laid-Open No. 64-65482).
A method for resolving scintillation components when the inorganic scintillator includes two scintillation components with different scintillation decay times will now be explained.
When the outputted scintillation pulse comprises two scintillation components with different scintillation decay times as mentioned above, the scintillation intensity I is represented by the following formula (A).I=Imax(ae−(t−tmax)/τ1+(1−a)e−(t−tmax)/τ2)  (A)In formula (A), I is the scintillation intensity, a is a variable, tmax is the time at which the scintillation intensity is Imax, t is the time elapsed from tmax, τ1 is the longer scintillation decay time and τ2 is the shorter scintillation decay time. Imax is as defined above.
In formula (A), the term represented by formula (B) below is the scintillation intensity I1 at time t for the scintillation component with the longer scintillation decay time, and the term represented by formula (C) below is the scintillation intensity I2 at time t for the scintillation component with the shorter scintillation decay time. Based on these formulas, the scintillation output I1in for the scintillation component with the longer scintillation decay time and the scintillation output I2in for the scintillation component with the shorter scintillation decay time are represented by the following formulas (D) and (E), respectively.I1=Imaxae−(t−tmax)/τ1  (B)I2=Imax(1−a)e−(t−tmax)/τ2  (C)
                              I                      1            ⁢                                                  ⁢            in                          =                              I            max                    ⁢          a          ⁢                                    ∫                              t                max                            ∞                        ⁢                          ⅇ                                                -                                      (                                          t                      -                                              t                        max                                                              )                                                  /                                  τ                  1                                                                                        (        D        )                                          I                      2            ⁢                                                  ⁢            in                          =                                            I              max                        ⁡                          (                              1                -                a                            )                                ⁢                                    ∫                              t                max                            ∞                        ⁢                          ⅇ                                                -                                      (                                          t                      -                                              t                        max                                                              )                                                  /                                  τ                  2                                                                                        (        E        )            
FIG. 2 is a graph schematically showing typical time-dependent change in the intensity of a scintillation pulse comprising two scintillation components with different scintillation decay times. The curve shown as the alternate dot-dash line (b) represents the time-dependent change in the scintillation intensity I1, the curve shown as the dash-dot-dot line (c) represents the time-dependent change in the scintillation intensity I2, and the curve shown as the solid line (a) represents the time-dependent change in the scintillation intensity I.
The waveform of the obtained scintillation pulse (solid line (a) in FIG. 2) may be fitted into formula (A) above for resolution into each of the scintillation components. In this case, the fitting is performed using the least square method, optimizing a, τ1 and τ2.
Thus, in order to shorten the scintillation decay time of an inorganic scintillator using a Gd-containing metal oxide as the matrix, it is sufficient to increase the Ce content ratio. Yet, Nuclear Instruments and Methods in Physics Research A404 (1998) 283, for example, teaches that a higher Ce content ratio in an inorganic scintillator tends to result in reduced scintillation output from the inorganic scintillator.